1. Field of the Invention
The present invention relates to an intake- and/or exhaust-valve timing control system which is optimally adapted for use in internal combustion engines. More particularly, this invention relates to a system which is variably capable of controlling the intake- and/or exhaust-valve timing depending upon the operating state of the engine, for example the magnitude of engine load and/or engine speed.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various intake- and/or exhaust-valve timing control systems for internal combustion engines for generating the optimal engine performance depending upon the operating state of the engine.
As is generally known, the valve timing is determined such that optimal engine performance is obtained; however the predetermined valve timing is not suitable under all operating conditions. As is well known, when the engine is operating within a range of low revolutions, higher torque will be obtained with an earlier intake/valve timing than the predetermined valve timing.
Such conventional intake- and/or exhaust-valve timing control systems for internal combustion engines have been disclosed in Japanese Utility Model (Jikkai) Showa 62-66206 and in U.S. Pat. No. 4,421,408. In these conventional valve timing control systems, a timing pulley is rotatably supported through a ring gear mechanism by the front end of a cam shaft. The ring gear mechanism includes a ring gear having an inner helical gear engaging an outer helical gear portion fixed on the front end portion of the cam shaft and an outer helical gear engaging an inner helical gear portion formed on the inner peripheral wall of the timing pulley. In this manner, the ring gear rotatably engages between the timing pulley and the cam shaft. The ring gear is normally biased in the axial direction of the cam shaft by spring means, such as a coil spring. Under this condition, the intake- and/or exhaust-valve timing is in general set to a predetermined valve timing. The conventional valve timing control system also includes an oil flow control valve assembly disposed in the cam shaft for controlling the flow of oil supplied to a pressure chamber defined between the timing pulley and the front end portion of the cam shaft and an electromagnetic actuator assembly attached to a rocker cover for operating the flow control valve. The flow control valve assembly and the electromagnetic actuator assembly are coaxially arranged with respect to each other. The actuator assembly employs a substantially cylindrical plunger which is normally biased to the innermost position thereof by the spring. The flow control valve assembly employs a substantially cylindrical spool valve which is normally biased to the outermost position, wherein the outer end of the plunger and the outer end of the spool valve abut each other, by means of a spring. When the actuator is activated, the plunger is shifted from the innermost position to the outermost position, and as a result the spool valve is moved from the outermost position to the innermost position, alternately moving between the oil supply passage and the oil exhaust passage, opening each passage in turn. In these constructions, when the actuator is excited, the flow control valve is controlled such that working fluid (operating oil) having high pressure, via the engine oil pump, is supplied from the oil pan through the flow control valve to the pressure chamber, and thus the ring gear is rotated and moved in an axial direction opposing the direction of the spring bias provided. Therefore, the phase angle between the timing pulley and the cam shaft is slightly changed, with the result that the valve timing relative to the crank angle is variably controlled. Conversely, when the actuator is deactivated, the flow control valve is controlled such that the working fluid is supplied from the oil pan through the control valve to the internal space defined by the cylinder head.
However, in conventional valve timing control systems, the spool valve of the flow control valve assembly is rotated relative to the plunger of the actuator assembly in a state wherein the facing ends of the spool valve in the flow control valve and the plunger of the actuator assembly continuously abut each other irrespective of whether or not the actuator is activated; therefore, abrasion occurs between the ends of the plunger and the spool valve. This results in reduction of the stroke of the spool valve. As a result, flow control to the ring gear mechanism may not be efficiently performed.
As previously described, since conventional valve timing control systems are designed such that the working fluid is discharged through the flow control valve into the internal space in the cylinder head, in internal combustion engines having positive crankcase ventilation (PCV) systems, a portion of the working fluid is mixed with the blow-by fumes and other vapors in the crankcase or in the cylinder head. Therefore, the working fluid may deteriorate. When the blow-by gas including a portion of the working fluid is introduced through the PCV valve which is provided to regulate the flow of blow-by gas from the crankcase, into the intake manifold, the blow-by gas is mixed with the incoming air-fuel mixture and sent to the cylinders for burning. As a result, the concentration of pollutants discharged from the exhaust valve into the atmosphere is increased, thereby reducing emission control performance. For this reason, internal combustion engines with conventional valve timing control systems require separators, such as a baffle plate and steel screen, for separating the working fluid from the blow-by fumes. These parts increase the overall cost of the engine.